1. Field of the Invention
This invention relates to a display apparatus for displaying characters, still images, moving images and other forms of information and it also relates to a video information display system for which such an apparatus can be suitably used.
2. Related Background Art
In recent years, a variety of handy apparatus adapted for use in multimedia environment have been developed and commercialized. Portable terminals and lap-top computers are among them. These apparatus can be used to retrieve information at locations other than offices and are also provided with capabilities of processing and transmitting information.
However, such apparatus are mostly designed for personal use and not adapted for teleconferences where a large number of attendants discuss subject matters in multimedia environment on the basis of commonly shared visual pieces of information. Thus, despite the development of such apparatus, people are forced to physically travel over a long distance at the cost of productivity in order to attend a meeting to be held at a single location.
Proposals have been made to "electronize" conferences and realize video conference systems. However, such systems are mostly designed for teleconferences attended by people stationed at different remote locations, where attendants can see each other and look at displayed data but cannot write-in and/or otherwise process data to update the information they have. Probably, this is mainly because no technology is currently available to instantly display updated information with a level of resolution and brightness comparable to that of the overhead projector (OHP) at a reasonable cost. Meanwhile, with a currently known visual telephone system, although two parties located at the opposite ends of the line can look at and talk to each other, neither of them can access the source of information possessed by the other party in order to write, erase and/or otherwise process data for the other party.
As for a projection type display apparatus, Japanese Patent Application Laid-open No. 54-139569 discloses a display apparatus comprising a light valve and a schlieren optical system for projecting an image as the light valve receives light. It additionally comprises a liquid crystal layer that produces a spheric and periodic structure that diffracts the flux of light directed to the light valve when a voltage is applied to the layer.
The above apparatus operates satisfactorily in terms of diffraction effect so long as the light valve is sufficiently large and a high degree of resolution is not required for it and, at the same time, the cell size of each pixel is large. To the contrary, however, when the light valve is relatively small and required to show an enhanced degree of resolution and still the cell size of each pixel is as small as tens of several micrometers, the diffracting zone is made extremely narrow and no sufficient amount of diffracted light becomes available to remarkably lower the level of brightness and contrast.
Japanese Patent Application Laid-open No. 61-167297 discloses a display apparatus comprising a plurality of liquid crystal panels arranged to a multilayer structure in order to display bright images on the screen. However, the apparatus is still unsatisfactory in terms of the absolute brightness of the displayed images mainly because three primary colors are overlaid one on the other for displaying color images.